Apparatus and method for forming an optical fiber

ABSTRACT

An apparatus for forming a optical fiber which includes a furnace for softening an optical fiber blank; a tractor for drawing the optical fiber from the softened optical fiber blank; and a first applicator for applying a coating of a first coating material to the optical fiber, the first applicator having a rotatable die.

This application is a continuation-in-part of application Ser. No.08/756,574 filed Nov. 26, 1996 by the same inventors now abandoned.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an optical fiber, and moreparticularly, to a optical fiber having spin.

2. Discussion of the Related Art

In recent years, long distance fiber optic communication has becomeincreasingly important. In long distance optical fibers, it is importantto retain the integrity of signals in the optical fibers. One problem inoptical fiber signal integrity is Polarization Mode Dispersion (PMD).

PMD is the broadening of a optical signal pulse in a single mode fiberdue to the dependence of the group velocity of the polarization state ofthe field, i.e., birefringence. In the case of constant birefringence,there are two states of polarization: one called “slow” and one called“fast”. For any impulse at all, the superposition of the two statescause a temporary increase in pulse width which grows linearly withdistance traveled. Therefore, the optical signal pulses will disperse,and the signals become unusable if the pulses combine.

In optical fibers, birefringence is caused by asymmetries andimperfections of the optical fibers such as ellipticity of the core andby anisotropies from internal stresses. A characteristic repeat lengthcan be generally associated to these asymmetries and imperfections,being the length after which, on average, said asymmetries andimperfections are reproduced. Typical values for said repeat length areof the order of few meters to few hundred of meters. In addition, notonly do fiber parameters vary, but also external stresses and geometricdeformations are introduced by spooling, cabling, or installation.

These stresses create random couplings between the polarization modes ofoptical fibers. Further, the continuous exchange of power between theslow and the fast states limits the expansion of the impulse to afunction related to the square root of the distance. Because thesestresses are random, PMD is characterized by statistics. Typically afiber's PMD is 0.05 to 0.5 ps km^(−½).

Further information, on birefringence and on PMD in optical fibers canbe found, e.g., in the following articles: W. Eickhoff et al. AppliedOptics, Vol. 20, No. 19 pp. 3428-3435 (1981) and A. F. JudyInternational Wire & Cable Symposium Proceedings, pp. 658-664 (1994).

According to the above, it is desirable to reduce PMD as much aspossible. There are two ways to reduce PMD: to reduce localbirefringence and to increase the power exchange between the twopolarization states.

To increase the power exchange, a method has been developed to apply atwist or a spin in the optical fiber. Twist refers to the rotation of avitrified optical fiber about its axis whereas spin refers to rotationof a molten optical fiber. Both processes are similar with respect totheir effects on the two polarization states. Furthermore, the twist orspin may be applied with turns constant in direction along the length ofthe optical fiber or with turns alternating in direction along thelength of the optical fiber.

The amount of rotation applied to a twisted or spun fiber ischaracterized by the twist, τ, which is defined by the number of turnsper unit length. If twist is high, in relation to the previouslymentioned repeat length, each of the two polarizations will bealternately in the slow and the fast states along fiber lengths shorterthan the typical perturbation lengths. This results in a continuous andhomogenous exchange of power between the two states, therebysignificantly reducing the PMD.

Typically, spun fibers require τ=1-10 turns/m to induce a birefringenceof β=1-10 m⁻¹ in order to overcome the effects of ellipticity andstress. When alternated twist is applied, the inversion period of thetwist, i.e. the distance required to alternate the direction of thetwist back and forth, is less critical and is typically 1-100 m.

The Applicant has afforded the problem of applying twist to the fiber inthe molten phase, in order that such twist be frozen in the glassstructure, when it is solidified in a cooling stage.

WO 83/00232, (Central Electricity Generating Board) discloses a methodof making an optical fiber comprising drawing the fiber from a heatedpreform whilst effecting continuous relative rotation between thepreform and the drawn fiber. To produce the spun fiber, the preform maybe rotated during the drawing process.

The Applicant has observed that the method of rotating the preformrequires the rotation of a large, potentially imbalanced, mass at highrotational velocity. For example, an optical fiber having an alternatingtwist of τ=1 turn/m and a draw speed of v_(draw)=10 m/s requires thepreform to rotate at 600 revolutions per minute. This can cause seriousproblems of vibrations in the fiber. As a result, the method is usuallyunsuitable.

U.S. Pat. No. 5,298,047, to Hart, Jr. et al., discloses that PMD can besubstantially reduced if, during drawing of the fiber, a torque isapplied to the fiber such that a “spin” is impressed on the fiber.Desirably the torque is applied such that the spin impressed on thefiber does not have a constant spatial frequency, e.g., has alternatelyclockwise and counterclockwise helicity. According to Hart, Jr. et al.,the torque advantageously is applied at a point downstream from thecuring station, and it is most preferred to apply the torque by means ofthe first guide roller. The guide roller can be caused to oscillate backand forth or to move back and forth axially.

The above prior art methods require the rotation of a preform or theapplication of a torque, by means of a guide roller, while drawing theoptical fiber.

The Applicant has observed that oscillation or movement of a guideroller requires a complex mechanical apparatus and can cause a relevantstress on the fiber coating just after its application and curing.Furthermore, conventional coating applications resist the transmissionof torque from the roller to the uncooled optical fiber in the vicinityof the neckdown area of the furnace, thereby reducing effectiveness ofthe method.

Coating of glass optical fibers is desirable for the chemical andphysical protection of the fibers. A common practice is to apply to theglass fiber a double-layer acrylic coating, whereby a first layer, witha relatively low elastic modulus, constitutes a “soft cushion” aroundthe fiber and a further layer, with a relatively high elastic modulus,protects the fiber from the environment.

To give the fiber a homogeneous protection, it is important that eachcoating layer is concentric with the glass fiber. Concentricity of alayer, defined as the ratio between the minimum and maximum thickness ofsaid layer in a section, is conveniently higher than 0.7, preferablyhigher than 0.85. Lower concentricity values correspond to a coatinglayer which is too thin on one side and, consequently, gives the fiberan insufficient protection. Increasing the coating layer thickness,while allowing low concentricity, might solve the protection problem,but would entail an increased bulkiness for the coated fiber and anincreased cost.

To improve concentricity of the coating, the Applicant has tried use ofa self-centering die, with a radial profile such as to cause high radialpressure, together with homogeneous feeding of the coating resin intothe die, so as to get a homogeneous pressure. Both these measures haveshown to be insufficient, as they can be made ineffective either by aslight lack of symmetry in the die or in the die holder, or by a slightlack of alignment of the drawing apparatus, both of which are difficultto avoid.

SUMMARY OF THE INVENTION

Accordingly, the present invention is directed to an apparatus andmethod for fabricating an optical fiber that substantially obviates oneor more of the problems due to limitations and disadvantages of therelated art.

An object of the present invention is to provide an apparatus withreduced complexity for fabricating an optical fiber having spin.

Another object of the present invention is to provide an apparatus and amethod for fabricating an optical fiber, wherein resistance to anapplied torque is reduced.

A further object of the present invention is to provide an apparatus anda method for drawing an optical fiber and coating it with one, or more,highly concentric coating layers.

The Applicant has found that a torque can be applied to a fiber byrotation of a die in a coating applicator.

A spin can thus be imparted on the optical fiber in the softened regionnear the neckdown area.

Further, the Applicant has found that a highly concentric coating can beapplied to the fiber, by rotating a die in a coating applicator.

Additional features and advantages of the invention will be set forth inthe description which follows, and in part will be apparent from thedescription, or may be learned by practice of the invention. Theobjectives and other advantages of the invention will be realized andattained by the structure particularly pointed out in the writtendescription and claims hereof as well as the appended drawings.

To achieve these and other advantages and in accordance with the purposeof the present invention, as embodied and broadly described, anapparatus for forming a optical fiber includes a furnace for softeningan optical fiber blank; means for drawing the optical fiber from thesoftened optical fiber blank; and a first applicator for applying acoating of a first coating material to the optical fiber, the firstapplicator having a rotatable die.

To advantage, the apparatus comprises rotating means associated withsaid rotatable die, such as an electro-magnetic, mechanical or hydraulicmotor, or a gas turbine.

The rotating means can be adapted to rotate the rotatable die in aconstant direction, or in alternating opposite directions.

Advantageously the rotatable die has a concave portion having a passagefor the optical fiber. It further advantageously comprises means forfeeding the first coating material to the concave portion of therotatable die.

In an embodiment, the rotatable die is of a pressurized type. In thisembodiment, the die has preferably a support and a pressurized concavemember surrounding the optical fiber and rotatably engaged to saidsupport. Rotatable sealing means are preferably provided between saidsupport and said pressurized concave member.

The first coating material is advantageously a viscous fluid.

Preferably the rotatable die turns about an axis along which the opticalfiber is drawn.

In an embodiment, the apparatus comprises a second applicator forapplying a coating of a second coating material to the optical fiber.The second applicator may include a second rotatable die, which may beof a pressurized type.

In a preferred embodiment, the apparatus comprises a cooling stage forcooling the optical fiber.

According to another aspect, the present invention relates to a methodfor forming an optical fiber which includes the steps of softening anoptical fiber blank; drawing an optical fiber from the softened opticalfiber blank; applying a coating of a coating material on the opticalfiber through a die; wherein said step of applying a coating furthercomprises rotating said die while said coating material flows around theoptical fiber.

Preferably rotation of said die causes said coating material to turn andthe turning of said coating material applies a torque to the opticalfiber.

In an embodiment, said die is rotated in a constant direction, and/orrotation of said die turns the optical fiber in a constant direction.

In an alternative embodiment, said die is rotated in alternatingdirections, and/or rotation of said die turns the optical fiber inalternating directions.

Preferably said method further comprises the step of cooling the drawnoptical fiber. Said step of cooling the drawn optical fiberadvantageously sets twist in the optical fiber.

In said method, the step of applying a coating may include applying afirst layer of a first coating material through a first die and a secondlayer of a second coating material, over said first layer, through asecond die, wherein at least one of said dies is rotated. According to apreferred embodiment, said first die is rotated. According to anotherembodiment, both said dies are rotated. In the latter case, said seconddie is preferably rotated in a same direction as said first die.

Preferably said die is rotated with a maximum rotation equal to, orgreater than, 5,000 rev/min. More preferably, said maximum rotation iscomprised between 10,000 and 30,000 rev/min, in association with a fiberdrawing velocity comprised between 1 and 10 m/s.

To the purpose of the present invention, maximum rotation indicateseither rotation speed, in case of constant and uniform rotation, or thepeak value of the rotation speed in case of alternate or non-uniformrotation.

According to a further aspect, the following invention has to do with amethod for reducing polarization mode dispersion in an optical fiber,comprising the steps of providing a partially softened portion of theoptical fiber and rotating a viscous fluid about a longitudinal axis ofthe fiber, and in contact therewith, whereby a torque is applied byviscous drag to said partially softened portion of the optical fiber.

Said viscous fluid can to advantage be a fiber coating material. Saidtorque can be applied by rotating a die through which said coatingmaterial is applied to the optical fiber.

It is to be understood that both the foregoing general description andthe following detailed description are exemplary and explanatory and areintended to provide further explanation of the invention as claimed.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, which are included to provide a furtherunderstanding of the invention and are incorporated in and constitute apart of this specification, illustrate embodiments of the invention andtogether with the description serve to explain the principles of theinvention. In the drawings:

FIG. 1 is a diagram of an apparatus for fabricating an optical fiber;

FIG. 2 is a diagram of a rotatable die for applying a coating inaccordance with the present invention;

FIG. 3 is a diagram of an applicator having a rotatable die inaccordance with the present invention;

FIG. 4 is a diagram of a pressurized rotatable die in accordance withthe present invention; and

FIG. 5 is a diagram of an optical fiber showing the rotation and torqueat various stages.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Reference will now be made in detail to the preferred embodiments of thepresent invention, examples of which are illustrated in the accompanyingdrawings.

The present invention applies spin to the optical fiber in the processof applying a coating to the optical fiber. A number of coating devicesare known in the art including Published Japanese Patent Application52-117935 to Ueno et al., U.S. Pat. No. 4,194,462 to Knowles, and U.S.Pat. No. 4,246,299 to Ohls. Ueno et al. discloses a thin film coatingapparatus for coating primer on a glass fiber wherein dust is preventedfrom attaching on the coating. Knowles teaches a coating assemblyincluding a recessed housing and a trust block wherein aligned passagesextend through each of the elements so that an optical waveguide can beinserted into the apparatus from the side rather than threaded endwisethrough the apparatus. Ohls discloses a method of coating an opticalwaveguide filament employing a die body having at least partly taperedcentral aperture and radial means for introducing coating material tothe central aperture. However, the systems of the above references arenot suitable for applying spin to an optical fiber.

FIG. 1 shows an apparatus in accordance with the present invention forfabricating an optical fiber 102. The apparatus comprises a furnace 104for softening an optical fiber blank 106, a cooling stage 108, a firstapplicator 110 for applying a first coating on the optical fiber 102, asecond applicator 112 for applying a second coating on the optical fiber102, and a tractor 114 for drawing the optical fiber 102 from thesoftened optical fiber blank 106.

The furnace 104 has a neckdown area 116 from which the optical fiber 102is drawn. The blank 106 is fed into the furnace using a movable means(not shown) so that the furnace at least partially surrounds the blank106. The blank 106 may be of any desired shape, but the blank 106 iscommonly an elongate cylinder or tube. Because the index of refractionof the optical fiber 102 generally varies radially, the index ofrefraction of the blank 106 in general also varies radially. Often, thisis achieved by forming the blank 106 as a rod-shaped core inside ashell, the core having a greater index of refraction. The front end ofthe blank 106 is softened so that it can be drawn into an optical fiber.The optical fiber 102 is drawn through the neckdown area 116, and thenthrough the cooling stage 108 to be cooled. By cooling the optical fiber102, any spin is frozen into the optical fiber 102. In drawing theoptical fiber 102, various parameters such as temperature, draw speedand blank size are controlled to precisely regulate the size of theoptical fiber 102.

The first and second applicators 110 and 112 include a die havingopenings through which the optical fiber 102 passes. In the first andsecond applicators 110 and 112, the optical fiber 102 passes through andis coated by a fluid coating material. The first and second applicatorsmay also have UV lamps to cure the optical coating after application.With respect to the cured coatings, the first coating is preferablysofter than the second coating. As an alternative to a first applicatorand a second applicator for applying a double layer of coating, a singleapplicator may be provided when application of a single layer of coatingto the optical fiber is appropriate. A number of materials are suitablefor the first and second coating materials, but generally resins arepreferred, in particular acrylic resins.

The tractor 114 provides the force in drawing the optical fiber 102 fromthe softened blank 106. Further, in the preferred embodiment, thetractor 114 determines the draw speed of the optical fiber 102. Finally,the completed optical fiber is stored on a take-up reel until it isused. The take-up reel may be integral with the tractor 114 or it may beseparate.

In a preferred embodiment, the first applicator 110 includes a rotatabledie 200 which is adapted to rotate about an axis through which theoptical fiber 102 passes. FIG. 2 shows a rotatable die 200 according toa preferred embodiment of the present invention.

As shown in FIG. 2, the rotatable die 200 includes a funnel-shapedstructure 202, and a reservoir 204. In a preferred embodiment, thereservoir is not eccentric. As the optical fiber 102 passes through thereservoir, the optical fiber is coated with a coating material containedtherein. In this manner, the first coating 206 is formed on the opticalfiber 102. The thickness of the first coating 206 is regulated by thesize of the exit aperture 208, the viscosity of the resin, and the drawspeed of the optical fiber 102.

To provide a spinning of the optical fiber 102, the die 200 is arotatable die as indicated the by arrow and dotted line of FIG. 2, andcan be set into rotation by appropriate rotation means, e.g.,mechanical, electric, magnetic, including, e.g., gears, belts, turbines,electromagnetic couplers, or by other known means. Rotatable sealingmeans may be conveniently located between the rotating die and the diesupport. As the rotatable die 200 turns, the resin also turns as aresult of the resin's viscosity. As the resin turns about the opticalfiber 102, the resin imparts a torque on the optical fiber 102.

Generally, the coupling between the turning of the rotatable die 200 andthe torque applied on the optical fiber 102 increases with viscosity ofthe resin. While a number of coating materials can be used for the firstcoating 206, resins such as Neorad NEP 58 and NEP 94, by Zeneca, London(GB) have been found to be suitable. NEP 58 has a viscosity of 5.2 Pa·sat 25° C. (1.9 Pa·s at 40° C.), and forms a coating having a tensilemodulus of 3.30 MPa. NEP 94 has a viscosity of 5.0 Pa·s at 25° C. (2.2Pa·s at 40° C.), and forms a coating having a tensile modulus of 3.4MPa.

As known, the majority of fluids used to coat optic fibers do not behaveas newtonian fluids and their viscosity may present complex phenomenasuch as thixotropy, viscoplasticity or pseudoplasticity. Thus, in manycases viscosity per se is inadequate to distinguish satisfactorily thecoating materials which will be able to apply a torque to an opticalfiber according to the present invention. In general, the coatingmaterials will be selected among those fluids which are able to apply atorque to an optical fiber which decreases PMD thereof to 0.1 ps/Km^(½)or less when said coating material is applied to said optical fiber bymeans of a rotatable die rotating at at least 5,000 rev./min. accordingto this invention. Preferably, the viscosity the coating material is ofat least 1 Pa·s at the temperature of the coating fluid entering intothe rotatable die. Still preferably, said viscosity value is from 2.1 to20 Pa·s. Even more preferaby said viscosity value is from 2.1 to 10Pa·s.

A spun optical fiber having an alternating twist of τ=1 turn/m and adraw speed of v_(draw)=10 m/s requires, e.g., maximum rotation at 600rev/min. The present invention can provide such rotations by rotatingthe rotatable die 200, e.g., with maximum rotations of 5,000 rev/min, ormore. As discussed, the rotatable die 200 is either turned in a constantdirection or turned in alternating directions.

Further, because the coating material applies the torque to the opticalfiber 102, the problem of the coating material resisting the torqueapplied by prior art rollers is overcome. Instead, the torque is morefreely transmitted through the optical fiber to the softened portionnear the neckdown area 116 of FIG. 1. After the softened optical fiber102 is spun near the neckdown area 116, the spin becomes set into theoptical fiber in the cooling stage 108.

Therefore, in accordance with the present invention, the optical fiberis fabricated as follows. An optical fiber blank is softened in afurnace. Then, the softened blank is drawn through a neckdown area ofthe furnace to form an optical fiber by a force provided by a tractor.Next, the optical fiber is cooled in a cooling stage. In this step, anyspin applied to the softened optical fiber is frozen into the opticalfiber. Then, the drawn optical fiber passes through and is coated byfirst and second applicators with first and second coatings,respectively. Preferably, the first applicator applies a soft coating,and the second applicator applies a hard coating. The first and secondcoatings may be resins, for example. In accordance with the presentinvention, a rotatable die of the first applicator is turned. As aresult, the rotation of the rotatable die causes the coating fluidcontained therein to also turn. The turning of the coating fluid aboutthe optical fiber causes a torque on the optical fiber, therebyimparting a spin on the optical fiber in the softened region near theneckdown area. In the described process, the turning direction of therotatable die may be constant or alternating.

Only rotation of the first die has been discussed so far. However, inthe cases where a second applicator is present, the second applicatormay comprise a rotatable die to impart a spin to the fiber by causing atorque to the fiber via the viscous resin of the second coating and thecured first coating. The turning direction of a rotatable die of thesecond applicator may be constant or alternating. Making both first andsecond die rotatable may involve a more complex mechanical structure,but the provision of two rotatable dies allows the application ofgreater torque to the fiber. In this case, the rotation speed of the tworotatable dies are adapted to the different viscosities of the twoliquid coating resins.

FIG. 3 shows an embodiment of a first applicator 110 having a rotatabledie 200A. As shown in FIG. 3, the first applicator 110 comprises a motor302, a shaft 304, a first bearing mechanism 306, a first gear 308, asecond gear 310, a second bearing mechanism 312, and a rotatable die200A. The motor 302 rotates in a constant direction if an optical fiberhaving twist of a constant direction is desired or in alternatingdirections if an optical fiber having twist of alternating directions isdesired. Shaft 304 is driven by motor 302. The shaft 304 is supportedand aligned by support 314 and first bearing mechanism 306. The firstbearing mechanism 306 allows the shaft 304 to freely rotate. First gear308 is fixedly mounted on shaft 304. First gear 308 drives second gear310 which is held in position by the second bearing mechanism 312 andsupport 314. The second bearing mechanism 312 allows the second gear 310to freely rotate. Furthermore, the bearing mechanism 306 and 312 of apreferred embodiment would minimize vibration and friction. The secondgear may include a second gear shaft 316. The second gear shaft 316 ishollow so that the optical fiber 102 can be drawn through the apparatus.The rotatable die 200A is mounted on the second gear 310.

The operation of the first applicator 110 will now be discussed. Themotor 302 drives the second gear 310 via shaft 304 and first gear 308.The rotatable die 200A is mounted on the second gear 310. Therefore, asthe second gear 310 is rotated, the rotatable die 200A, containing aviscous liquid coating material, also rotates. As a result, the liquidcoating material turns according to the rotation of the rotatable die200A. Meanwhile, the optical fiber 102 is drawn through the firstapplicator 110. Specifically, the optical fiber 102 passes through therotatable die 200A and the second gear 310. Accordingly, the firstliquid coating material flows around the optical fiber 102. As a result,a torque is applied to the optical fiber 102. Further, the optical fiber102 is coated with the first coating material after passing through thefirst applicator 110

A rotatable die of the non-pressurized type has been so far described.In its place, a pressurized rotatable die can be conveniently used, asschematically shown in FIG. 4. Parts of FIG. 4 corresponding to parts ofFIG. 3 have been denoted by the same numeral. A rotatable die 400 ismounted on gear 310 through gearing mechanism 312, anchored to support314. The rotatable die includes a hollow gear shaft 318, having passagesfor an optical fiber 102 at its upper and lower portions and having ahollow chamber 320 in communication with a die 200A. A supply duct for acoating liquid is provided through support 314, to feed a chamber 324,extended around a middle portion of gear shaft 318. Chamber 324 is incommunication with chamber 320 through gear shaft 318, via passages 326.Leakage from chamber 324 is prevented by convenient rotatable sealingmeans 328, providing a low friction sealing between support 314 and gearshaft 318, so as to allow free rotation of gear shaft 318.

In operation, a liquid coating material is fed under pressure from duct322 to chamber 320, through chamber 324 and passages 326. Leakage fromthe upper passage for the optical fiber through gear shaft 318 isprevented from happening by choosing a chamber pressure that can bebalanced by the suction caused by fiber 102 while it is dragged throughthe same passage.

As would be evident to one skilled in the art, the rotation of therotatable die can be achieved by other mechanisms. For example, gears308 and 310 can be replaced with a belt system or the rotatable die maybe mounted directly to a motor shaft. In addition, the motor 302 may beof any type, e.g., electro-magnetic, or mechanical, or hydraulic, or agas turbine. Accordingly, any apparatus for turning the rotatable die200A can be used. Moreover, in applications where a second rotatable dieis used, the apparatus similar to that of FIG. 3 could be used to rotatethe second rotatable die. Furthermore, the second rotatable die mayinclude a pressurized die.

The principles of spinning the optical fiber the rotation of a rotatabledie containing a viscous coating material will now be explained. FIG. 5shows the process of spinning an optical fiber in accordance with thepresent invention.

In FIG. 5, axial position is designated by z, and rotation of theoptical fiber, solidified below the neckdown area of the furnace, isdesignated by θ=θ(z). The origin of the z axis is taken at a point wherethe fiber has reached a stable diameter (typically 125 μm), such thatθ(0)=θ₀. The primary applicator is located at z₁ and θ(z₁)=θ₁. In thesame way, the secondary applicator will be located at z₂ and θ(z₂)=θ₂.Finally, the tractor is located at z₃ and θ(z₃)=θ₃.

As has been described, torque can be applied to the fiber throughrotation of the rotatable die that applies the first coating, aspreviously shown with reference to FIG. 2. For Newtonian fluids whichare, to a close approximation, acrylic resins with a low molecularweight, the flow can be described as the linear superposition of twoflows: a draw flow, which is associated with the fiber's axial velocityand involves the draw of the resin, and a flow resulting from therelative rotation between the fiber and the rotatable die.

The rotatable die can be modeled as a cylinder of effective radius R₁and height h₁, which rotates with an angular velocity Ω_(f) imposed fromoutside. Therefore, the flow from the rotation is essentially a Couetteflow between two cylinders, the external one comprising the rotatabledie and the internal one comprising the fiber of radius r₀ which, as aresult of the viscous forces of the resin and the elastic torsion of thefiber, rotates at angular velocity dθ₁/dt. The torque M₁ (position z₁)is given by $\begin{matrix}{M_{1} = {{4{\pi\mu}_{1}{h_{1}\left( {\Omega_{f} - \frac{\theta_{1}}{t}} \right)}\frac{r_{0}^{2}}{1 - \left( \frac{r_{0}}{R_{1}} \right)^{2}}} = {k_{1}\left( {\Omega_{f} - \frac{\theta_{1}}{t}} \right)}}} & 1\end{matrix}$

where μ₁ is the viscosity of the resin at the application temperature.In the second applicator, which is here considered as being non-rotated,a torque will be developed which is connected to the local rotationdθ₂/dt of the fiber alone. With notation analogous to the above, thetorque M₂ (position z₂) is given by $\begin{matrix}{M_{2} = {{4{\pi\mu}_{2}h_{2}\frac{\theta_{2}}{t}\frac{r_{1}^{2}}{1 - \left( \frac{r_{1}}{R_{2}} \right)^{2}}} = {k_{2}\frac{\theta_{2}}{t}}}} & 2\end{matrix}$

where r₁ is the radius of the first coating.

The torque due to the viscous rotation in the neckdown area is afunction of the rotational velocity $\begin{matrix}{M_{0} = {K_{0}{\frac{\theta_{0}}{t}.}}} & 3\end{matrix}$

The constant K₀ can be evaluated by considering that through theneckdown area the torque must be constant in the presence of a rotationof angular velocity ω(z). Therefore, in any section with constant z, thetorque, in cylindrical coordinates, at the neckdown area is$\begin{matrix}{M_{0} = {2\pi {\int_{0}^{R{(z)}}{\tau_{z\quad \theta}r^{2}\quad {r}}}}} & 4\end{matrix}$

where τ_(z)θ is a component of the stress tensor and R(z) is the radiusof the molten preform at z. Assuming homogeneous rotation at everysection, one has $\begin{matrix}{v_{\theta} = {{\omega (z)}r}} & 5 \\{and} & \quad \\{\tau_{z\quad \theta} = {{- {\mu (z)}}\frac{\partial v_{\theta}}{\partial z}}} & 6\end{matrix}$

where μ(z) is the local viscosity of glass, depending on temperature.Integrating with respect to r, a differential equation for the angularvelocity is derived: $\begin{matrix}{M_{0} = {\frac{\pi}{4}{R(z)}^{2}{\mu (z)}\frac{\omega}{z}}} & 7\end{matrix}$

with boundary conditions: ω(preform)=0; ω(fiber)=dθ₀/dt. Therefore,after integration, $\begin{matrix}{M_{0} = {\frac{\pi}{4}\frac{1}{\int_{0}^{L_{nd}}\frac{z}{{R(z)}^{2}{\mu (z)}}}\frac{\theta_{0}}{t}}} & 8\end{matrix}$

where L_(nd) is the length of the region in which the neckdown takesplace (i.e., the passage from the preform radius to the fiber radius).Comparison of equations 3 and 8 gives the value of K₀.

To obtain the profiles of radius and viscosity, R(z) and μ(z),respectively, it is necessary to take into account the transportequations that describe the drawing process (see, e.g., R. B. Bird, W.E. Stewart, E. N. Lightfoot: “Transport Phenomena”, John Wiley) If, forexample, V_(draw) is 10 m/s, draw tension is 100 g, R(preform) is 3 cm(typical process parameters), it results K₀=2×10⁻⁴ dyne·cm/(rev/min).

Since fiber portions are now being considered in which the optical fiberis solid and can be modeled as being essentially elastic, therelationship between torque M(z) and twist dθ/dz is given (see, e.g., L.Landau, E. Lifchitz “Theorie de l'élasticité”) $\begin{matrix}{{M(z)} = {{\frac{\theta}{z}\frac{\pi}{4}\frac{E}{1 + v}r_{0}^{4}} = {H\frac{\theta}{z}}}} & 9\end{matrix}$

where E is Young's modulus for the optical fiber (72,000 MPa for glass),υ is Poisson's ratio (0.4 for glass), and r₀ is the radius of the fiber(e.g., 0.0625 mm). With these parameter values, H≈6.16 dyne·cm/(rad/m).

Ignoring fiber inertia in rotation, the sum of the torques must be zeroat every point. With respect to FIG. 5, $\begin{matrix}\begin{matrix}{{{{- k_{2}}\frac{\theta_{2}}{t}} + {H\left( {\frac{\theta_{1} - \theta_{2}}{z_{2} - z_{1}} - \frac{\theta_{2} - \theta_{3}}{z_{3} - z_{2}}} \right)}} = 0} \\{{{k_{1}\left( {\Omega_{f} - \frac{\theta_{1}}{t}} \right)} + {H\left( {{- \frac{\theta_{1} - \theta_{2}}{z_{2} - z_{1}}} + \frac{\theta_{0} - \theta_{1}}{z_{1}}} \right)}} = 0} \\{{{{- K_{0}}\frac{\theta_{0}}{t}} - {H\frac{\theta_{0} - \theta_{1}}{z_{1}}}} = 0.}\end{matrix} & 10\end{matrix}$

An implicit assumption is that H is the same along the length of theoptical fiber (from 0 to z₁) and for the optical fiber with one (partlycrosslinked) coating (z₁ to z₂) or with two coatings (the outer beingpartly crosslinked) (z₂ to Z₃). This assumption is generally acceptableas the Young's modulus for the coating is very low in comparison to thatof glass.

In the preferred embodiment it will be sought to have M₂<<M₁ in order toavoid excessive torque in the elastic portion of the optical fiber. Theprevious condition can be met, in a preferred embodiment, by choosingthe viscosity μ₁ of the first liquid coating substantially higher thanthe viscosity μ₂ of the second liquid coating as, e.g., in the case ofthe experiments described in the following, where μ₂ is more than doublethan μ₂. If μ₁ is substantially higher than μ₁, provision of a rotatabledie in the second coating applicator is expected to give only a smallefficiency improvement in coupling torque to the fiber. If, on thecontrary, μ₂ is about equal to, or less than, μ₁, a rotatable dieprovided in the second applicator might be particularly effective inimparting torque to the fiber. In the above, the skilled in the art canconsider using a rotatable die in a second applicator, if the operatingconditions for a single rotatable die, such as coating viscosity, dierotating speed, and torque to be applied, are found to be critical inrelation to the sought fiber drawing velocities or other processparameters. Further, the skilled in the art can select the coatingmaterial (or materials) in order that its (their) viscosity is betteradapted to the operating conditions, in accordance with the teaching ofthe present invention. In the following example it will be assumed thatthe condition M₂<<M₁ applies. In that case, $\begin{matrix}{\frac{\theta_{1} - \theta_{2}}{z_{2} - z_{1}} = {\frac{\theta_{2} - \theta_{3}}{z_{3} - z_{2}} = {\frac{\theta_{1} - \theta_{3}}{z_{3} - z_{1}}.}}} & 11\end{matrix}$

Further, K₀ is small, so that θ₀≡θ₁. The angle θ₃ represents theresidual rotation in the tractor, in spite of the torque provided byfriction. The rotatable die is conveniently capable of transferring thehighest torque when θ₃=0. In the following calculations, this conditionis assumed to be held. A non-zero θ₃ would give as a disadvantage apartial residual torque of the reeled fiber, torque which in any casewould not be permanent and could be relaxed in successive phases ofrewinding. Therefore, in this case $\begin{matrix}{\Omega_{f} = {\frac{\theta_{0}}{t} + {\theta_{0}\frac{H}{k_{1}\left( {z_{3} - z_{1}} \right)}}}} & 12\end{matrix}$

As an example, to obtain a given level τ of maximum twist and a givennumber n of maximum fiber turns, at a draw velocity v_(draw), analternated rotation

θ₀=2 πn sin(Γ_(n) t) for n=1, 2, 3, . . .   13

can be applied to the fiber by appropriate alternated rotation of therotatable die, where Γ_(n) is $\begin{matrix}{\Gamma_{n} = {\frac{\tau \cdot v_{draw}}{2\pi \quad n}.}} & 14\end{matrix}$

A complete period of rotation inversion is equal to $\begin{matrix}{T_{n} = {\frac{2\pi}{\Gamma_{n}} = {\frac{4\pi^{2}n}{\tau \cdot v_{draw}}.}}} & 15\end{matrix}$

Therefore, by integrating the angular velocity, one has that therotation of the rotatable die should be $\begin{matrix}{\theta_{f} = {2\pi \quad {{n\left( {{\sin \left( {\Gamma_{n}t} \right)} - {{\cos \left( {\Gamma_{n}t} \right)}\frac{H}{\Gamma_{n}{k_{1}\left( {z_{3} - z_{1}} \right)}}}} \right)}.}}} & 16\end{matrix}$

As an example, a process is performed with a draw speed of v_(draw)=10m/s to obtain a maximum twist τ=1.5 turn/m and one complete turn of thefiber (n=1); the inversion frequency is Γ=5π rad/s and the period is 0.4s; the first coating fluid has a viscosity μ₁=5 Pa s; the geometry ofthe rotatable die is such that h₁=4 mm and R₁=120 mm; the die hasconstant k₁=0.0135 dyne·cm/ (rad/s); and the distance from the rotatabledie to the tractor is z₃−z₁=5 m. Then, the rotation of the rotatable die(in rad/s) is given by: θ_(f)(t)=2π[sin(5 πt)−5.82 cos(5 πt)]. Further,the maximum angular velocity is Ω_(f, max)≈570 rad/s≈5400 rev/min. Themaximum torque at the rotatable die is M_(1, max)≈7.7 dyne·cm.

Another important consideration is the elastic torque of the fiber,$\begin{matrix}{\frac{\theta_{1} - \theta_{3}}{z_{3} - z_{1}},} & 17\end{matrix}$

between the coating applicator, where active torque is applied, and thetractor. This torque should not be excessive in order to avoid breaks inthe optical fiber and because it would require high angular velocitiesin the rotatable die. Preferably the residual elastic torque of thefiber, if any, is less than 5 turn/m, more preferably less than 2turn/m.

Experiment 1

In a first experiment performed by the Applicant, an apparatus fordrawing an optical fiber as previously described was used. Inparticular, the die was a pressurized rotatable die as previouslydescribed with reference to FIG. 4. The following parameter values wereselected:

V_(draw)=7.25 m/s

μ₁=5-10 Pa·s

r₁=95 μm

μ₂=2-5 Pa·s

r₂=125 μm.

It is to be noted that the resin temperature in the die locallyincreases, due to viscous friction caused by die rotation and by axialmovement of the fiber. The given ranges of values for the resinviscosities correspond to the temperatures of the resin in the die.

The experiment was repeated with non-rotating die and with die havingnon-zero rotation Ω_(f), either constant or alternated with alternatingfrequency Γ.

Results are summarized in the following table, where PMD was measuredfor fiber lengths of 1 km, drawn from one single preform, in the givenorder, under different die rotation conditions, as indicated.

Primary Secondary coating coating Ω_(f) Γ PMD concentr concentr(rev/min) (Hz) (ps/km^(½)) icity icity 0 0 0.211 0.5 0.83 22,000 1,50.092 0.88 0.85 0 0 0.201 0.69 0.81 22,000 2 0.054 0.86 0.85 12,000 00.132 0.63 0.82

As it can be seen from the table, rotation of a die gave fibers withlower PMD, or higher coating concentricity, or both, in comparison withfibers coated by a non-rotating die. It is further observed thatalternated die rotation resulted in relatively lower PMD values.

The high speed rotation of the liquid coating outside the applicator,before curing, could in principle cause problems in the coatingapplication, e.g. lack of adhesion between the coating and the glassfiber. However, the Applicant has found that no such problem arises.Direct microscopic observation of samples of coated fiber produced inthe experiment has shown a good coating adhesion to the glass fiber and,in general, a coating quality similar to that resulting from applicationby a standard, non-rotating die. This has been confirmed by microscopicobservation of the coated fiber after immersion of the fiber into waterat 60° C. for a period of 30 days. It is believed that the actingcentrifugal forces are offset by the atmospheric pressure and by thesurface tension of the liquid coating.

Experiment 2

In a second experiment performed by the Applicant, with the sameapparatus used during the first experiment, but with a differentpreform, the following parameter values were selected:

v_(draw)=4 m/s

μ₁=5-10 Pa·s

r₁=95 μm

μ₂=2-5 Pa·s

r₂=125 μm

The experiment was repeated while giving the die an alternated rotation,with two values of maximum rotation Ω_(f) and with alternating frequencyΓ=2 Hz.

Results are summarized in the following table:

Primary Secondary coating coating Ω_(f) Γ PMD concentr concentr(rev/min) (Hz) (ps/km^(½)) icity icity 11,000 2 0.064 0.85 0.85 22,000 20.073 0.97 0.82

Low PMD values and a high coating concentricity are obtained in bothcases.

Experiment 3

In a third experiment performed by the Applicant, with the sameapparatus and the same parameter values as indicated for the firstexperiment, fibers were drawn from eight different preforms, in eachcase both with non-rotating die and with alternately rotating die. Inthe latter case, maximum rotation was Ω_(f)=10000 rev/min andalternating frequency was Γ=4 Hz. Samples of 1 km length were cut fromthe fibers drawn from the preforms while the die was non-rotating. ThePMD of each sample was measured: the average PMD was 0.205 ps/km^(½),with a standard deviation of 0.068 ps/km^(½). A total of 25 samples,each of a length of 1 km, were cut from the fibers drawn from thepreforms while the die was rotating, and the corresponding PMD measured.In this case the average PMD was 0.077 ps/km^(½), with a standarddeviation of 0.014 ps/km^(½). The maximum PMD for said 25 samples waslower than 0.10 ps/km^(½).

Therefore, a high quality optical fiber having spin is obtained inaccordance with the present invention. By rotating a rotatable dieduring the application of an optical fiber coating, the troublesomeprocedure of creating spin by rotating a preform or by oscillating ormoving a guide roller is avoided. Further, a torque is applied withreduced resistance. That is, torque is more efficiently and moredirectly applied to the softened portion of the optical fiber in theneckdown area of a furnace.

Further, by the present invention coating concentricity is improved;still further, coating concentricity is made high and stable even in thepresence of a lack of symmetry in the die or in the die holder, or inthe presence of a lack of alignment of the drawing apparatus.

It will be apparent to those skilled in the art that variousmodifications and variations can be made in the apparatus and method forfabricating an optical fiber of the present invention without departingfrom the spirit or scope of the invention. Thus, it is intended that thepresent invention cover the modifications and variations of thisinvention provided they come within the scope of the appended claims andtheir equivalents.

What is claimed is:
 1. Optical fiber coating apparatus for providing acoated optical fiber with an axis and a spin with respect to the axissufficient to provide Polarization Mode Dispersion (PMD) which is notgreater than 0.1 ps/Km^(½), said apparatus comprising: a furnace forsoftening an optical fiber blank; a tractor for drawing an optical fiberfrom the softened optical blank and advancing the optical fiber along apath extending from the optical fiber blank, the optical fiber beingsoftened adjacent to the softened optical fiber blank; and spin applyingapparatus for applying spin to the softened optical fiber adjacent tothe softened optical fiber blank, said spin applying apparatuscomprising: at least one rotatable coating applicator extending aroundthe path of the optical fiber and spaced from the softened optical fiberadjacent the softened optical fiber blank in the direction of thetractor for applying a coating material to the optical fiber; a coatingmaterial in the applicator, the coating material having a viscosity ofat least 1 Pas at the temperature at which the coating material issupplied to the applicator; rotating means coupled to the rotatablecoating applicator for rotating the applicator and thereby transmittingtorque to the optical fiber through the coating material applied to theoptical fiber and applying spin to the softened optical fiber adjacentthe softened optical fiber blank, the rotating means being adapted torotate the rotatable coating applicator at a rotation speed whichprovides said spin; and a cooling stage disposed intermediate thesoftened optical fiber and the rotatable coating applicator for coolingfiber in the optical fiber path between the softened optical fiber andthe rotatable coating applicator and thereby freezing the spin in theoptical fiber which has been produced in the softened optical fiber byrotation of the rotatable coating applicator.
 2. The apparatus accordingto claim 1 characterized in that the rotating means is adapted to rotatethe coating applicator at a rotation speed of at least 5000 revolutionsper minute.
 3. The apparatus according to claim 2 characterized in thatthe tractor is adapted to draw said fiber at a drawing speed of from1-10 meters per second and the rotating means is adapted to rotate thecoating applicator at a rotation speed of from 10,000 to 30,000revolutions per minute.
 4. The apparatus according to claim 1 whereinthe viscosity of the coating material is from 2.1 to 20 Pas.
 5. Theapparatus according to claim 1 wherein the viscosity of the coatingmaterial is from 2.1 to 10 Pas.
 6. The apparatus according to claim 1,characterized in that said rotating means comprises an electro-magnetic,mechanical or hydraulic motor, or a gas turbine.
 7. The apparatusaccording to claim 6, characterized in that said rotating means isadapted to rotate said coating applicator in a constant direction. 8.The apparatus according to claim 6, characterized in that said rotatingmeans is adapted to rotate said coating applicator in alternatingopposite directions.
 9. The apparatus according to claim 1,characterized in that said coating applicator has a concave portionhaving a passage for the optical fiber.
 10. The apparatus according toclaim 9 comprising means for feeding said coating material to saidconcave portion of said coating applicator.
 11. The apparatus accordingto claim 1, characterized in that said coating applicator is of apressurized type.
 12. The apparatus according to claim 11, characterizedin that said coating applicator comprises a support and a pressurizedconcave member surrounding the optical fiber and rotatably engaged tosaid support.
 13. The apparatus according to claim 12, characterized inthat rotatable sealing means are provided between said support and saidpressurized concave member.
 14. The apparatus according to claim 1,further comprising a second coating applicator for applying a coating ofa second coating material to the optical fiber.
 15. The apparatusaccording to claim 14, characterized in that the second coatingapplicator is disposed along the path of the optical fiber intermediatesaid one rotatable coating applicator and the tractor, wherein thesecond coating applicator extends around and is rotatable around theoptical fiber path, and further comprising a source of the secondcoating material coupled to the second coating applicator for supplyingthe second coating material to the second coating applicator as theoptical fiber is advanced along said path.
 16. The apparatus accordingto claim 15, characterized in that said second coating applicator is ofa pressurized type.